The present invention relates to a polishing apparatus, in particular, a semiconductor wafer polishing apparatus.
For example, in the fabrication of semiconductor devices, it is usually necessary for a semiconductor wafer to be subject to a precise polishing process known as chemical-mechanical planarization ("CMP") to polish and planarize a top surface of the wafer. In the polishing process, a top surface of a wafer carried by a carrier is brought into a contact with a polishing surface of a resilient polishing pad provided on a turntable under a pressure while a slurry is supplied onto the polishing surface, whereby the top surface of the wafer is mechanically and chemically polished and planarized
Accordingly, it is desired that the pressure between the top surface of the wafer and the polishing surface of the polishing pad is uniform all over the top surface of the wafer. However, it is generally difficult to attain and/or maintain such a uniform pressure, mainly due to unevenness of the polishing surface of the polishing pad, whereby it becomes difficult to planarize the top surface of the wafer to a desirable extent.
A main cause of such unevenness in the polishing surface resides in a relationship of engagement between a top surface of a wafer which is commonly in the shape of a circle and a polishing surface of a resilient polishing pad. The wafer is engaged with the polishing surface of the resilient polishing pad at a position offset from the turning center of the polishing pad on a turntable and thus along a circular ring-shaped path on the polishing surface of the turning polishing pad. Accordingly, among the points on the polishing surface which engage with the wafer, a point on a circle about the center of the polishing surface which goes through the center of the wafer is pressed for the longest time per turn of the polishing pad, and points circles about the center of the polishing surface which go through the radially innermost and outermost points of the wafer relative to the center of the polishing pad are pressed for the shortest time, and vice versa for the time periods during which those points are freed from pressing by the wafer to restore their initial levels. As a result, the surface of the polishing pad which is brought into engagement with the wafer becomes uneven, whereby the pressure between the wafer and the polishing surface varies depending on radial engaging positions therebetween.
To solve such a problem, it was devised that a wafer carrier is provided with an air pressure chamber and a flexible seal member defining the bottom of the pressure chamber which supports a wafer on the outer surface thereof, whereby the wafer is yieldably urged against a polishing pad by the air pressure in the pressure chamber. However, it is difficult for this method to adequately cope with the uneveness of the polishing surface which may be formed
Japanese Laid-Open Patent Application H9-225812 (JP -209225812A) discloses another method for solving the above-noted problem in which a pressing device is provided at a position spaced from a wafer carrier so as to press an area of a polishing surface of a polishing pad which is to be brought into engagement with the wafer carried by the wafer carrier with a pressure equal to or greater than the pressure between the wafer and the polishing surface of the polishing pad so that the pressing device planarizes the area to attain a uniform pressure between the wafer and the polishing pad. However, various causes other than that above-noted are considered to relate to differences in pressures between a polishing surface of a polishing pad and a wafer and it is difficult for the method disclosed in the above-noted Japanese Application to appropriately cope with the problem of differences in pressures between the wafer and the polishing surface.